1. Field of the Invention
The present invention relates to a liquid crystal composition in which a cholesteric phase is formed, a liquid crystal display element using the same, and electronic paper including the same.
2. Description of the Related Art
Recently, development of electronic paper is active in enterprises, universities, and etc. As markets considered to have promising applications of electronic paper, various applied portable apparatus have been proposed, including electronic books first of all, sub-displays of mobile terminal apparatus, and display parts of IC cards. A promising display method for electronic paper employs a display element utilizing a liquid crystal composition in which a cholesteric phase is formed (a cholesteric liquid crystal). A cholesteric liquid crystal has excellent characteristics such as semi-permanent display holding characteristics (memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
FIG. 10 schematically shows a sectional configuration of a liquid crystal display element 51 capable of full-color display using cholesteric liquid crystals. The liquid crystal display element 51 has a structure in which a blue (B) display portion 46b, a green (G) display portion 46g, and a red (R) display portion 46r are stacked in the order listed from the side of a display surface. In the figure, the display surface is provided on the side where a top substrate 47b is provided, and outside light (indicated by the arrow in a solid line) impinges on the display surface from above the substrate 47b. An eye of a viewer and the viewing direction (indicated by the arrow in a broken line) are schematically illustrated above the substrate 47b. 
The B display portion 46b has a liquid crystal layer 43b for blue (B) enclosed between a pair of top and bottom substrates 47b and 49b and a pulse voltage source 41b for applying a predetermined pulse voltage to the liquid crystal layer 43b for B. The G display portion 46g has a liquid crystal layer 43g for green (G) enclosed between a pair of top and bottom substrates 47g and 49g and a pulse voltage source 41g for applying a predetermined pulse voltage to the liquid crystal layer 43g for G. The R display portion 46r has a liquid crystal layer 43r for red (R) enclosed between a pair of top and bottom substrates 47r and 49r and a pulse voltage source 41r for applying a predetermined pulse voltage to the liquid crystal layer 43r for R. A light absorbing layer 45 is disposed on a bottom surface of the bottom substrate 49r of the display portion 46r for R.
The cholesteric liquid crystals used in the liquid crystal layers 43b, 43g, and 43r for B, G, and R are liquid crystal mixtures obtained by adding a chiral additive (also called a chiral material) in a relatively great amount, i.e., at a content of several tens wt %, to a nematic liquid crystal. When a nematic liquid crystal contains a relatively great amount of chiral material, a cholesteric phase, which is a strong helical twist of a layer of nematic liquid crystal molecules, can be formed. A cholesteric liquid crystal is also referred to as a chiral nematic liquid crystal.
A cholesteric liquid crystal has bistability (memory characteristics), and the liquid crystal can be put in either planar state or focal conic state by adjusting the intensity of an electric field applied to the same. When the liquid crystal once goes to the planar state or the focal conic state, it stays in the state with stability even when there is no electric field. The planar state is obtained by applying a predetermined high voltage between top and bottom substrates 47 and 49 to apply an intense electric field to a liquid crystal layer 43 and abruptly making the electric field zero thereafter. The focal conic state can be obtained, for example, by applying a predetermined voltage lower than the high voltage between the top and bottom substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and abruptly making the electric field zero thereafter.
The display principle of the liquid crystal display element utilizing cholesteric liquid crystals will now be described with reference to the B display portion 46b by way of example. FIG. 11A shows alignment of liquid crystal molecules 33 of the cholesteric liquid crystal observed when the liquid crystal layer 43b for B of the B display portion 46b is in the planar state. As shown in FIG. 11A, the liquid crystal molecules 33 in the planer state are sequentially rotated in the direction of the thickness of the substrates to form a helical structure, and the helical axis of the helical structure is substantially perpendicular to substrate surfaces.
In the planar state, light rays having predetermined wavelengths in accordance with the helical pitch of the liquid crystal molecules are selectively reflected by the liquid crystal layer. A wavelength λ at which the maximum reflection occurs is expressed by λ=n·p where n represents the average refractive index of the liquid crystal layer and p represents the helical pitch.
Therefore, when blue light is to be selectively reflected by the liquid crystal layer 43b for B of the B display portion 46b in the planar state, the average refractive index n and the helical pitch p are determined, for example, such that λ=480 nm is true. The average refractive index n can be adjusted through selection of the liquid crystal material and the chiral material, and the helical pitch p can be adjusted by adjusting the content of the chiral material.
FIG. 11B shows alignment of the liquid crystal molecules 33 of the cholesteric liquid crystal observed when the liquid crystal layer 43b for B of the B display portion 46b is in the focal conic state. As shown in FIG. 11B, the liquid crystal molecules 33 in the focal conic state are sequentially rotated in in-plane directions of the substrates to form a helical structure, and the helical axis of the helical structure is substantially parallel to the substrate surfaces. In the focal conic state, the liquid crystal layer 43b for B loses the selectivity of wavelengths to reflect, and most of incident rays are transmitted. Since the transmitted rays are absorbed by the light-absorbing layer 45 disposed on the bottom surface of the bottom substrate 49r of the R display portion 46r, a dark (black) state can be displayed.
As thus described, in the case of the cholesteric liquid crystal, the reflection and transmission of light can be controlled by the alignment of the liquid crystal molecules 33 which are helically twisted. The liquid crystal display element 51 capable of full-color display is fabricated by enclosing a cholesteric liquid crystal selectively reflecting green or red light in the planar state in the liquid crystal layer 43g for G and the liquid crystal layer 43r for R, respectively, in the same manner as for the liquid crystal layer 43b for B as described above.
FIG. 12 shows examples of reflection spectra of the liquid crystal layers 43b, 43g, and 43r in the planar state. The abscissa axis represents wavelengths (nm) of reflected light, and the ordinate axis represents reflectance (in comparison to a white plate; %). The spectrum of reflection in the liquid crystal layer 43b for B is indicated by the curve connecting the symbols ▴ in the figure. Similarly, the spectrum of reflection in the liquid crystal layer 43g for G is indicated by the curve connecting the symbols ▪, and the spectrum of reflection in the liquid crystal layer 43r for R is indicated by the curve connecting the symbols ♦.
As shown in FIG. 12, the center wavelengths of the reflection spectra of the liquid crystal layers 43b, 43g, and 43r in the planar state have magnitudes in an ascending order of B, G, and R. Therefore, the helical pitches of the cholesteric liquid crystals in the liquid crystal layers 43b, 43g, and 43r have magnitudes ascending in the order in which the layers are listed. Thus, the chiral material contents of the cholesteric liquid crystals in the liquid crystal layers 43b, 43g, and 43r must descend in the order in which the liquid crystal layers are listed.
In general, a greater twist must be given to the liquid crystal molecules to shorten the helical pitch thereof, the shorter the wavelengths to be reflected. Accordingly, the chiral material content of the cholesteric liquid crystal increases. There is another general tendency that an increase in a chiral material content necessitates an increase in a driving voltage. Further, a reflection bandwidth Δλ increases with refractive index anisotropy Δn of a cholesteric liquid crystal.
Patent Document 1: JP-A-2003-147363
Patent Document 2: JP-A-2004-2765
A color liquid crystal display element having an RGB multi-layer structure utilizing cholesteric liquid crystals has a problem in that it is liable to degrading of the balance of color reproduction ranges and reduction in contrast. The balance of color reproduction ranges and contrast is significantly affected by scattering of light in a layer(s) in the dark state or focal conic state. For example, let us assume that any one of the liquid crystal layers is in the planar state and the liquid crystal layers for the remaining two colors are in the focal conic state. Then, when light is significantly scattered in the liquid crystal layers in the focal conic state, components of light scattered in the liquid crystal layers in the focal conic state are added to light reflected by the liquid crystal layer in the planar state as noises. As a result, the color purity of the displayed color is reduced. While all of the RGB liquid crystal layers are in the focal conic state when black is displayed, the density of the black is significantly reduced when light is significantly scattered in the liquid crystal layers. That is, the displayed image has a low contrast, and the display is blurred.
It is considered that a physical property dominating the scattering of light in a liquid crystal layer in the focal conic state is refractive index anisotropy Δn specific to the liquid crystal material. FIGS. 13A and 13B show relationships between refractive index anisotropy Δn and reflection of light in a liquid crystal layer. FIG. 13A shows a relationship between the refractive index anisotropy Δn and the brightness of reflected light in the planer state. The abscissa axis represents the refractive index anisotropy Δn, and the ordinate axis represents brightness (in comparison to a white plate; %). FIG. 13B shows a relationship between the refractive index anisotropy Δn and scattering of light in the focal conic state. The abscissa axis represents the refractive index anisotropy Δn, and the ordinate axis represents scattering (in comparison to a white plate; %).
As shown in FIGS. 13A and 13B, the liquid crystal layer has a higher refractive index in the planar state, the greater the Δn value. Although the brightness of the display screen of the liquid crystal display element is therefore improved, the scattering of light in the liquid crystal layer in the focal conic state is worsened at the same time. On the contrary, the scattering of light in the liquid crystal layer in the focal conic state is reduced at a smaller Δn value. However, since the refractive index of the liquid crystal layer in the planar state is also reduced, the brightness of the display screen is reduced. Thus, the relationship between the brightness and scattering of reflected light is a trade-off, and it is therefore difficult to achieve high brightness of the display screen and low scattering at the same time only by controlling the Δn value.
Patent Document 1 discloses a technique for providing cholesteric liquid crystals of liquid crystal layers for R, G, and B by mixing R and S materials, which are two optical isomers of chiral materials having different optical rotations, at different mixing ratios such that the same amount of the chiral materials is added in the liquid crystal layers for R, G, and B. However, even when the same amount of chiral materials is added in the liquid crystal layers for R, G, and B to provide the layers with the same physical properties such as Δn, the liquid crystal layers have different light scattering characteristics. It is therefore difficult to improve the color balance and contrast of the display screen satisfactorily.